1. Technical Field
The present invention relates to a color separation optical system for separating incident light into a plurality of color light components, and to an image pickup apparatus having the color separation optical system.
2. Related Art
Generally, image pickup apparatuses, such as a television camera and a video camera, have color separation optical systems. FIG. 17 illustrates an example of the configuration of a color separation optical system. The color separation optical system 101 separates incident light L, which is incident thereon via a taking lens 102, into three color components that are a blue light component LB, a red light component LR, and a green light component LG. Image pickup devices 4B, 4R, and 4G, such as charge coupled devices (CCDs), respectively used for the light components LB, LR, and LG, into which incident light is separated by the color separation optical system 101, are disposed at places respectively corresponding to the color light components LB, LR, and LG. The color separation optical system 101 is referred to as a Philips type color separation optical system, and is provided with a first prism 110, a second prism 120, and a third prism 130, which are arranged along an optical axis Z1 in order from the side of incidence of light. The blue light component LB, the red light component LR, and the green light component LG are taken out by the first prism 110, the second prism 120, and the third prism 130, respectively.
A blue-reflecting dichroic film DB1 is formed on a reflection/transmission surface 111 of the first prism 110. A red-reflecting dichroic film DR1 is formed on a reflection/transmission surface 121 of the second prism 120. The first prism 110 and the second prism 120 are disposed so that the surface 111 of the first prism 110, on which the blue-reflecting dichroic film DB1 is formed, and a surface of the second prism 120, on which light is incident, are spaced by an air gap 110 AG and face each other. Further, a trimming filter 151 is provided on a surface of the first prism 110, from which light exits. A dichroic film 151A is formed on a surface of the trimming filter 151, from which light exits. Similarly, a trimming filter 152, on which a dichroic film 152A is formed, is provided on a surface of the second prism 120, from which light exits. A trimming filter 153, on which a dichroic film 153A is formed, is provided on a surface of the third prism 130, from which light exits. The trimming filters 151, 152, and 153 are provided in order to make the spectral characteristic of the color separation optical system approximate an ideal characteristic. The trimming filters 151, 152, and 153 have a role of shaping spectral characteristics represented by wavelength components, which are not sufficiently shaped by the blue-reflecting dichroic film DB1 and the red-reflecting dichroic film DR1.
FIG. 19 illustrates ideal spectral characteristics of a color imaging system, which respectively correspond to a red component (R-component), a blue component (B-component), and a green component (G-component). Incidentally, the ideal spectral characteristics illustrated in FIG. 19 are normalized so that the maximum value of each of the R-component, the B-component, and the C-component is 1. The “ideal spectral characteristic” can be obtained by converting the chromaticity coordinates of three primary colors of a color reproduction medium thereinto and performing a linear transformation of a color matching function in an XYZ color coordinate system. Incidentally, the “color reproduction medium” is a medium for reproduction (or display) of an image taken by an image pickup apparatus. The “color reproduction medium” is, e.g., a display apparatus such as a liquid crystal monitor and a projector. FIG. 18 illustrates an example of chromaticity coordinates of three primary colors R, G, and B for obtaining an ideal characteristic. Three primary colors R, G, and B determine a color range that can be reproduced by a color reproduction medium.
Ideal color reproduction can be achieved in a case where a characteristic, which is the same as an ideal characteristic as illustrated in FIG. 19, can be obtained using a color separation optical system 101 illustrated in FIG. 17. However, actually, it is difficult to obtain a characteristic which is completely the same as an ideal characteristic. Thus, a color separation optical system is designed to obtain a characteristic which approximates an ideal characteristic. The color separation optical system 101 is designed so that a characteristic, which approximates an ideal characteristic, is obtained by appropriately adjusting the dichroic films DB1 and DR1 respectively formed on the prisms and the dichroic films 151A, 152A, and 153A respectively formed on the trimming filters 151, 152, and 153. FIG. 20 illustrates an example of color separation optical system obtained by performing such a design.
FIG. 21 illustrates examples of the designs of the dichroic films DB1 and DR1 used in the color separation optical system 101. As illustrated in FIG. 21, films having characteristics, the wavelength-to-transmittance characteristic curves of which have steep leading edges or trailing edges as compared with those of the characteristic curves of the ideal characteristics illustrated in FIG. 19, are used as the dichroic films DB1 and DR1, respectively. Additionally, unnecessary wavelength components of light exiting from an exit surface of each of the prisms are cut off using the trimming filters 151, 152, and 153 on which the dichroic films 151A, 152A, and 153A are formed, respectively.
Thus, characteristics are arranged using various trimming filters in the color separation optical system. For example, Patent Document 1 (JP-A-2005-208256) has proposed a method for improving color reproducibility by increasing the luminance level of a fresh color using a trimming filter which has a special spectral transmission characteristic. In addition, another method is known, which arranges a transmission characteristic by disposing a half mirror on a bonding surface between the second prism 120 and the third prism 130, instead of the dichroic film DR1, and providing dichroic films having transmission characteristics, which approximate ideal characteristics, as the trimming filters 152 and 153. FIG. 22 illustrates a spectral characteristic of a color separation optical system, which is made by a special arrangement to approximate an ideal characteristic.
However, the color separation optical system using a trimming filter provided with a dichroic film on an exit surface of a prism has a wavelength range, in which a reflectance is high in some wavelengths, as a characteristic of the dichroic film. Consequently, the above color separation optical system has problems in that multiple reflections occur between the dichroic surface and an imaging surface and result in occurrence of ghost flare, and that picture quality is degraded. FIG. 23 illustrates multiple reflections occurring at the side of an exit surface of the third prism 130, from which green light LG is taken out, in the color separation optical system 101, by way of example. As illustrated in FIG. 23, the image pickup device 4G has an imaging surface 401G, a cover glass 402, and an extraction electrode 403. For example, a part of green light LG passing through the trimming filter 153 for green light is reflected by the imaging surface 401G. Then, the return light reflected therefrom is reflected by the dichroic film 153A provided on the trimming filter 153 according to the wavelength selectivity characteristic of the dichroic film 153A. Thus, multiple reflection light 160 is generated to thereby cause ghost flare. Accordingly, hitherto, it is difficult to implement an imaging system having an ideal spectral characteristic, which reduces ghost flare.
Further, although FIG. 17 illustrates the Philips type color separation optical system 101 in which the first prism 110 and the second prism 120 are disposed by interposing the air gap 110 AG therebetween, particularly, a gapless type color separation optical system, which is provided without the air gap 110AG therebetween, has a problem in that polarization separation is liable to occur at a reflection dichroic surface. FIG. 24 illustrates an example of the configuration of the gapless type color separation optical system 101A. The gapless type color separation optical system 101A shown in FIG. 24 differs from the Philips type color separation optical system 101 illustrated in FIG. 17 in the order in which the color components are taken out therefrom. The color separation optical system 101A is configured so that green light LG, blue light LB, and red light LR are respectively taken out by the first prism 110, the second prism 120, and the third prism 130, respectively. In the color separation optical system 101A, a green-reflecting dichroic film DG1 adapted to reflect green light LG and to transmit blue light LB and red light LR is formed on a reflection/transmission surface 111 of the first prism 110. Moreover, a blue-reflecting dichroic film DB1 adapted to reflect blue light LB and to transmit red light LR is formed on a reflection/transmission surface 121 of the second prism 120. The surface 111 of the first prism 110, on which the green-reflecting dichroic film DG1 is provided, and an incidence surface of the second prism 120, on which light is incident, are closely attached to each other without providing an air gap therebetween.
In the case of such a gapless type color separation optical system 101A, an incidence angle θ of incidence of light on the reflection dichroic surface (blue-reflecting dichroic film DB1) of the second prism 120 increases, as compared with the Philips type color separation optical system 101. In a case where the incidence angle θ increases, a phenomenon called a “polarization separation” occurs at the blue-reflecting dichroic film DB1. Thus, as is understood from an example of a transmission characteristic curve illustrated in FIG. 25, in the blue-reflecting dichroic film DB1, a wavelength width W100 required to change the transmittance of the film DB1 from a low transmittance band to a high transmittance band has a tendency to increase in a case where the transmittance is assumed to be an average of the transmittance of P-polarized light and that of S-polarized light. It is considered to be necessary for obtaining an ideal spectral characteristic to set the wavelength width W100 in an appropriate wavelength range.
The invention is accomplished in view of such problems. An object of the invention is to provide a color separation optical system and an image pickup apparatus, which are enabled to improve color reproducibility by obtaining a characteristic which approximates an ideal spectral characteristic in view of influence of polarization separation caused according to the magnitude of an incidence angle.